This invention relates to digitally gain controllable amplifiers, and more particularly, a digitally gain controllable amplifier controllable by an analog gain control signal, and having an on-chip decoder and programmable gain distribution.
In a typical xe2x80x9chomodynexe2x80x9d receiver, a variable gain amplifier (VGA) may be used to maintain a constant signal amplitude to a load independent of the input signal amplitude. This variable gain amplifier may be implemented with either an analog variable gain amplifier, or a digital variable gain amplifier.
Using an analog variable gain amplifier has many disadvantages such as the requirement of complicated circuitry for maintaining a linear-in-dB gain control characteristic, as well as large chip area requirements for the temperature compensation and predistortion circuit that is used to control the analog variable gain amplifier. Furthermore, the predistortion and temperature compensation circuits consume a significant amount of supply current. Additionally, the analog gain controlled VGAs are noisy as the signal-to-noise ratio at the output of the amplifier depends on the gain setting. Further, analog gain controlled amplifiers essentially constitute amplitude modulators as the gain of the variable gain amplifier could vary as the instantaneous noise voltage overlaying the voltage control signal varies.
Because the digital gain controlled VGAs overcome many of the disadvantages of the analog gain controlled VGAs, it is desirable to use digital gain controlled VGAs. However, digital gain controlled VGAs have one major disadvantage: a required isolation of greater than 60 dB between the serial digital gain control word inputs and the RF signal input of the digital gain controlled VGA, in order to keep a spur generated at the signal frequency at the same level as the noise floor. This attenuation requirement is difficult to achieve when taking into account the parasitic effects that occur within the chip""s package and on the board that the chip is attached to.
Additionally, in a digital gain controlled VGA, the digital gain control word sent to the digital VGA must be in a specific format for use by the amplifiers present in the digital VGA. This generally requires that the digital gain controlled VGA be used only in conjunction with specific supporting external circuitry.
Further, while designing a radio receiver utilizing a typical digital gain controlled VGA having multiple amplifying circuits, the operating conditions for the receiver must be predicted in advance in order to set the gain distribution of the multiple amplifier circuits to achieve optimal noise and linearity performance. However, because the operating conditions for the radio receiver are constantly changing, this gain distribution does not yield optimal noise and distortion performance for the majority of the time that the receiver is operating.
The present invention is directed to overcoming one or more of the problems discussed above in a novel and simple manner.
In accordance with the invention, there is disclosed a digitally gain controllable amplifier controllable by an analog gain control signal. There is further disclosed a digitally gain controllable amplifier having an on-chip decoder. Additionally, there is disclosed a digitally gain controllable amplifier having a programmable gain distribution.
It is an object of the invention to provide a method of controlling a digital variable gain amplifier (DVGA) of a receiver where the DVGA includes an amplifier circuit, the method including the steps of receiving an analog gain control signal indicating a desired gain for the DVGA, and converting the analog gain control signal to a digital gain control signal. The method further includes applying the digital gain control signal to the amplifier circuit to achieve the desired gain, where the amplifier circuit has a gain controlled by the digital gain control signal.
It is a feature of the invention that the DVGA includes a plurality of amplifier circuits, and the method includes decoding the digital gain control signal into a plurality of gain settings, where each gain setting corresponds to one of the plurality of amplifier circuits. The method then includes applying each gain setting to a corresponding amplifier circuit in the DVGA to achieve the desired gain. It is an additional feature of the invention that the digital gain control signal is a final digital gain control signal and an initial digital gain control signal is determined by gain control circuitry, and the initial digital gain control signal is converted to the analog gain control signal. In a further preferred embodiment, the DVGA includes a plurality of amplifier circuits and the method includes controlling the noise and linearity characteristics of the DVGA by changing the distribution of gain across the plurality of amplifier circuits.
It is another object of the invention to provide a method of controlling a DVGA where the DVGA has a plurality of amplifier circuits and the gain of each amplifier circuit is controlled by a gain setting, the method including determining the desired noise and linearity characteristics of the DVGA for a time period. The method also includes increasing the gain setting of a first amplifier circuit, while decreasing gain settings for at least one of a remainder of the plurality of amplifier circuits in the DVGA if an improved/increased noise characteristic is desired for the time period. The method further includes decreasing the gain setting of the first amplifier circuit, while increasing the gain settings for at least one of the remainder of the plurality of amplifier circuits in the DVGA if an increased linearity characteristic is desired for the time period.
It is a feature of the invention that a received signal strength (RSS) is determined for a received signal where the desired noise and linearity characteristic is set for decreased noise if the RSS does not exceed an RSS threshold, and the desired noise and linearity characteristic is set for increased linearity if the RSS exceeds the RSS threshold. It is an additional feature of the invention that the bit error rate (BER) of a detected signal may be used in addition to the RSS where the determined noise and linearity characteristics are maintained if the BER does not exceed a BER threshold, and if the BER exceeds the BER threshold, the desired noise and linearity characteristics are set according to the RSS. It is an additional feature of the invention that an analog gain control signal is received indicating a desired gain for the DVGA, and the analog gain control signal is converted to a digital gain control signal. In a further feature of the invention, a gain control signal is received and encoded into a plurality of gain settings where each gain setting corresponds to one of the plurality of amplifier circuits.
It is an object of the invention to provide a DVGA where the DVGA has an amplifier circuit controlled by a digital gain control signal, the DVGA applying a desired gain to a received signal from a signal source and applying the amplified received signal to the load. The DVGA also includes an analog-to-digital (A/D) converter coupled to the amplifier circuit for receiving an analog gain control signal and converting the analog gain control signal to the digital gain control signal and applying the digital gain control signal to the amplifier circuit.
It is a feature of the invention wherein the DVGA comprises a plurality of amplifier circuits, and further comprises a decoder connected between the A/D converter and the plurality of amplifier circuits for decoding the digital gain control signal into a plurality of gain settings, where each gain setting corresponds to one of the plurality of amplifier circuits. It is another feature of the invention where the digital gain control signal is a final digital gain control signal and a digital-to-analog (D/A) converter external to the DVGA and coupled to the A/D converter receives an initial digital gain control signal from a gain control circuit and converts the initial digital gain control signal to the analog gain control signal. In this case, a common reference voltage may be used for the D/A converter and the A/D converter. Alternatively for this case, a latch may be coupled to the AND converter for latching the final digital gain control signal. The latch and the common reference may be used in conjunction with one another. It is a further feature of the invention that the DVGA comprises a plurality of amplifier circuits and a noise and linearity control (NLC) circuit is coupled to the DVGA for controlling noise and linearity characteristics of the DVGA by changing the distribution of gain across the plurality of amplifier circuits.
It is yet another object of the invention to provide a DVGA with improved noise and linearity characteristics where the DVGA includes a plurality of amplifier circuits, with each amplifier circuit having a gain controlled by a digital gain control signal, and applying a desired gain to a received signal from a signal source, and applying the amplified received signal to a load. The DVGA further includes an NLC circuit comprising a controller coupled to the plurality of amplifier circuits for determining a desired noise and linearity characteristic for the DVGA and including means for increasing a gain setting of a first amplifier circuit and decreasing gain settings for at least one of a remainder of the plurality of amplifier circuits if a decreased noise characteristic is desired, and means for decreasing the gain setting of the first amplifier circuit of the DVGA while increasing the gain settings of at least one of the remainder of the plurality of amplifier circuits if an increased linearity characteristic is desired in the DVGA.
In a feature of the invention, the DVGA includes a decoder for decoding a gain control signal into a plurality of gain settings for the plurality of amplifier circuits using a lookup table, and applies the gain settings to each of the plurality of amplifier circuits to achieve a desired gain, and the controller decreases the noise characteristic and increases the linearity characteristic by changing the values of the gain settings in the lookup table. It is a further feature of the invention that the DVGA further includes an RSS indicator coupled to the DVGA wherein the controller sets the noise and linearity characteristics of the DVGA for decreased noise if it is determined that an RSS does not exceed an RSS threshold, and the controller sets the noise and linearity characteristics of the DVGA for increased linearity if it is determined that the RSS exceeds the RSS threshold. In a further feature of the invention, the DVGA includes a bit error rate detector coupled to the DVGA for determining a bit error rate (BER) for a detected signal wherein the controller maintains the determined noise and linearity characteristics if the BER does not exceed a BER threshold, and if the BER exceeds the BER threshold, the controller sets the desired noise and linearity characteristics using the RSS indicator. In an additional feature of the invention, a lookup table is coupled to the NLC circuit where the lookup table is indexed by a digital gain control signal value and includes gain settings for the plurality of amplifier circuits. In this case, the means for increasing and the means for decreasing gain settings includes the NLC circuit changing the gain settings in the lookup table.
It is another object of the invention to provide a DVGA integrated circuit including at least one amplifier circuit, where each amplifier circuit has a gain controlled by a digital gain control signal and applies a desired gain to a received signal from a single source, and applies the amplified signal to a load. The DVGA further includes a decoder coupled to the at least one amplifier circuit, where the decoder includes a controller for determining a desired gain for the DVGA, and a memory coupled to the controller and having a memory table indexed by a gain control value, where each gain control value has a corresponding gain setting for the at least one amplifier circuit to allow the DVGA to achieve the desired gain. The decoder controller determines the gain control value from a received gain control signal and applies the gain settings corresponding to the gain control value from a memory table to the at least one amplifier circuit to achieve the desired gain.
It is a feature of the invention that the gain control signal of the DVGA integrated fo circuit is an analog gain control signal, and the DVGA includes an A/D converter coupled to the decoder which converts the analog gain control signal to a digital gain control signal, and a decoder controller uses the digital gain control signal to determine a gain control value. It is a further feature of the invention that the DVGA integrated circuit further includes an NLC circuit coupled to the decoder wherein the controller, responsive to the NLC circuit, changes the gain settings in the memory table to control the noise and linearity characteristics of the DVGA.